Aerostructures or airfoils generally possess a streamlined surface constructed such that air or other fluids flowing over the surface produce a pressure differential between generally opposing faces of the airfoil. This pressure differential applied across the airfoil surface produces a force, commonly called lift. Lift may describe a force generated in any direction in any medium. However, for most airfoils lift is typically the sum of forces applied to the airfoil perpendicular to direction of external flow around the airfoil. A second aerodynamic parameter that is associated with lift is lift induced drag, which is in the same vector direction as the external flow around the aerostructure. These properties and others are generally applicable to airfoils and aerostructures moving through a wide range of different fluid media including air and water.
The lift and drag characteristics of an airfoil are typically a function of the physical geometry of the airfoil, the characteristics of the external flow including velocity and density, and the orientation of the airfoil to the external flow. For an airfoil with a fixed structure the lift and drag characteristics are determined by the angle of the airfoil, or pitch, which effectively controls the relative angle of attack of the airfoil. The angle of attack for an airfoil is defined as the angle between the wing chord and the direction of the relative wind. On a traditional airfoil, a number of devices can be used to alter the relative lift-drag characteristics of the airfoil by effectively modifying the physical structure of the airfoil, including hinged surfaces such as ailerons, flaps, and slats, as well as, physical changes to the airfoil itself in the form of wing warping or discrete perturbations. By modulating the pitch and the physical characteristics of the airfoil or aerostructure, it is possible to modulate the lift and drag characteristics of the airfoil, and thus control the vehicle.
There are multiple ways to determine the lift of a particular airfoil or aerostructure. One method is through a mathematical construct called circulation. Circulation is effectively the line integral of the velocity of the air (or other fluid), in a closed loop around the boundary of an airfoil. Once an estimate of the circulation of air around an airfoil is known, then the sectional lift of the airfoil or other aerostructure in a particular section is calculated as the product of the fluid density, the freestream velocity, and the circulation. Although circulation is not the most intuitive method of understanding the actual mechanics of lift on an arbitrary airfoil, it provides a useful construct for understanding how changes in the flow field around an airfoil can be manifest as significant changes in the aerodynamic performance of the airfoil.
A circulation controlled airfoil or aerostructure may utilize a number of different techniques to modify the circulation of air around the airfoil directly, thus modulating the effective lift of the airfoil. One exemplary method of circulation control is the use of blowing or suction slots strategically placed on an airfoil. The controlled injection or removal of fluid, e.g., air, into and from the flow field around the airfoil produces a change in the circulation which in turn is used to manipulate the aerodynamic coefficients of the airfoil without the need to either physically alter the airfoil nor adjust the airfoil pitch.
Airfoils or aerostructures are used in a variety of applications. For example, on high speed automobiles it is common to use adjustable airfoils which, when coupled with the body itself, produces a lifting force to press the vehicle towards the ground in order to improve traction. Watercraft and submarines also use control of the lift-drag characteristics of airfoils to the control the vehicle as it passes through the water. Perhaps one of the most recognizable applications of airfoils and aerostructure control is in aircraft and gliders, where the wings, empennage, and even the body, engine nacelles, and nose are commonly manipulated using a variety of techniques including hinged control surfaces and physical modification of the structures in order to obtain controlled flight of the air vehicle.
A well-known application for airfoils is on helicopters or rotorcraft. A helicopter is a vehicle which uses rotating airfoils, commonly called blades or rotor blades, to generate a significant portion of the lift necessary for the vehicle to stay in flight. On many helicopters today, the pitch of the rotor blades is controlled in order to modulate the effective lift generated, using a complex mechanical system that enables the blades to be collectively manipulated as a group and for each individual blade to be manipulated usually as a function of rotational position. Helicopters need this control because as the rotating blades travel around the helicopter, the relative air velocity over the surface of the blades increases and decreases continuously based on whether the blades are heading into the flight path of the helicopter or away. Since the air velocity directly influences the lift generated by an airfoil, it is necessary to modulate the lift generated as a function of rotor position in order to maintain relatively constant lift throughout the rotation of the rotor blade and to minimize asymmetric forces applied to the rotor hub itself, thereby reducing overall stress in the assembly. Traditionally, blade lift properties have been controlled using a swash plate that a portion of the blades ride along to rotate their effective pitch as they rotate about the hub. The use of mechanical controls to change the pitch of an entire blade as it rotates about the hub is difficult to maintain and can result in large complex structures. The manipulation of a blade at the hub using mechanical means, like a swash plate, also induces significant stresses in the blade structure. Therefore, there is a need for a rotor blade having an alternative means for controlling the effective lift generated by the airfoil without the need to physically adjust the pitch of the entire blade. Several types of helicopters also use other lifting structures to generate additional lift, however the basic operation, principals, and challenges associated with the operation of the rotor blades remains essentially the same.
In addition to control of the main rotor blades, a helicopter also needs to control the reaction torque generated by the rotation of the rotor blades. In most helicopters, this is controlled via a small rotor system attached to the tail oriented such that the lift generated by the tail rotor counteracts the reaction torque caused by driving the main rotor. The lift generated by this rotor system is modulated by either changing the rotational velocity of the tail rotor or changing the pitch of individual blades. Other systems include the use of reaction jets and ducted fans. Other alternative approaches have attempted to control the lift generated by the tail boom of the helicopter generated by the downwash of air from the rotor blades over the tail boom aerostructure. Still other designs have incorporated a twin, counter-rotating main rotor to eliminate the torque.
These exemplary systems demonstrate only a small number of the variety of ways in which the ability to directly manipulate the effective lift generated by an airfoil or aerostructure traveling through a variety of fluid media, including air and water, can be used. Therefore, there is a need for a system and method of manipulating and controlling the aerodynamic characteristics of an airfoil or aerostructure. There is also a need for a system and method of manipulating and controlling the aerodynamic characteristics of an air vehicle control with either no or with significantly limited requirements for hinged control surfaces. Specifically, a system and method are needed to control the lift characteristics of an airfoil or helicopter rotor blade enabling either a reduction in the amount of motion or pitching required from the swash plate and the blade hinge system or to allow the complete elimination of the swash plate and blade hinges altogether. In the case of rotorcraft, directly manipulating and controlling aerodynamic coefficients can minimize the forces imposed on traditional rotor systems and enable the creation of hingeless rotors.